Absorption of aerosols above clouds from POLDER/PARASOL measurements and estimation of their Direct Radiative Effect
Abstract. The albedo of clouds and the aerosol absorption are key parameters to evaluate the direct radiative effect of an aerosol layer above clouds. While most of the retrievals of above clouds aerosol characteristics rely on assumptions on the aerosol properties, this study offers a new method to evaluate aerosol and cloud optical properties simultaneously (i.e. aerosol and cloud optical thickness, aerosol single scattering albedo and angström exponent). It is based on multi-angle total and polarized radiances both provided by the A-train satellite instrument POLDER – Polarization and Directionality of Earth Reflectances. The sensitivities brought by each kind of measurements are used in a complementary way. Polarization mostly translates scattering processes and is thus used to estimate the scattering aerosol optical thickness and the aerosol size. On the other hand, total radiances, together with the scattering properties of aerosols, are used to evaluate the absorption optical thickness of aerosols and the cloud optical thickness. In addition, a procedure has been developed to process the shortwave direct radiative effect of aerosols above clouds based on exact modeling. Besides the three case studies (i.e. biomass burning aerosols from Africa and Siberia and Saharan dust), both algorithms have been applied on the South East Atlantic Ocean and results have been averaged through August 2006. The mean direct radiative effect is found to be 33.5 W m−2. Finally, the effect of the heterogeneity of clouds has been investigated and reveals that it affects mostly the retrieval of the cloud optical thickness and not much the aerosols properties. The homogenous cloud assumption used in both the properties retrieval and the DRE processing leads to a slight underestimation of the DRE.